Illicit drugs are used by about 200 million people around the world, according to a study published on Friday in The Lancet.
It estimates that in 2009 between 149 and 271 million people used an illegal drug.
Cannabis users comprised between 125 and 203 million; users of opioids (heroin and morphine), amphetamines or cocaine totalled 15 to 39 million; and those who injected drugs numbered between 11 and 21 million.
"Cannabis use is associated with dependence and mental disorders, including psychoses, but does not seem to substantially increase mortality," it says.
"Illicit opioid use is a major cause of mortality from fatal overdose and dependence," it says, adding that injecting drug users faced specific threats of catching HIV and hepatitis by sharing needles.
All three types of drugs seem to be associated with higher rates of mental disorders, road accidents and violence, although information about this is often sketchy.
The estimate is based on data from the UN Office on Drugs and Crime (UNODC), from national surveys and peer-reviewed studies into the impact of drug use.
Ecstasy, LSD, non-medical use of prescription drugs and anabolic steroids are not included in the estimate.